The measurement of intraocular pressure is an important function performed by ophthalmologists and other eye care professionals. Pressure measurements are performed (a) as a routine part of the complete eye examination to identify patients with or at risk for developing glaucoma, (b) to monitor progress and response to treatment in patients with glaucoma and ocular hypertension.
The first tonometer was developed in 1926 and is called the Schiotz tonometer. This simple instrument employs a weighted plunger which is lowered onto an anesthetized eye. The amount of deflection of an indicator is proportional to intraocular pressure; however it is also sensitive to scleral rigidity which could lead to an inaccurate measurement. The intraocular pressure is obtained indirectly using a supplementary table. This somewhat difficult-to-use and inaccurate instrument is still popular today among older eye physicians and in general medical practice.
The Goldman applanation tonometer was developed in 1957 to measure intraocular pressure using an applanation method. The anesthetized cornea is flattened against a glass plate of known diameter, producing a meniscus of tear film between the head of the instrument and the cornea. This technique is less sensitive to scleral rigidity. However, the Goldman tonometer must be attached to a slit-lamp microscope so that the manual measurement can be made accurately.
There is a portable version of the Goldman tonometer known as the Perkins tonometer which is a hand held device employing similar applanation technology. However, this instrument is quite difficult to use as the examiner's eye must be literally within inches of the patient's eye and stabilization of the instrument is difficult. Therefore, except for examinations under anesthesia, the Perkins tonometer is rarely used.
The McKay/Marg tonometer, introduced in 1959, exploits different technology. This instrument incorporates a small electrical strain gauge in the tip of the hand-held probe which is attached to a large carrying case containing an amplifier, strip chart recorder, and transformer. This is a contact device and therefore requires the use of topical anesthesia. The instrument works by relating a change in voltage to intraocular pressure. The user interprets the strip chart output signal, usually interpolating over several subjectively "acceptable" signals.
The Pneumotonometer was introduced in 1975. It works by bringing a small air burst toward the cornea. The back pressure is sensed, and is found to be proportional to intraocular pressure. This instrument seems to have inaccuracies, especially at the low range.
Another instrument by A. O. Reichert utilizes an air applanation technique, which does not require touching the instrument to the eye. An air puff of a given force and diameter is used to flatten the cornea. The amount of flattening is sensed by the machine and is proportional to pressure. This is the most popular unit in the optometric community because it does not require topical anesthesia.
Harold Rose and Bruce Sand developed an applanation tonometer which utilizes a digital read-out and is described in U.S. Pat. No. 3,724,263.
Although some of the above instruments provide reliable estimates of intraocular pressure, they lack portability, reliability, accuracy, or acceptance in the marketplace, and none of the instruments are ideal in providing some key features. Therefore, eye care professionals and the general medical community is still seeking a precise hand held portable tonometer to assist them in the diagnosis and management of glaucoma.